An electroluminescent element is a luminescent device which takes advantage of the electroluminescence of a solid fluorescent substance. Inorganic electroluminescent elements employing an inorganic luminescent material have been put to practical use, and some of these are used as the back lights of liquid-crystal displays and in applications such as flat displays. However, inorganic electroluminescent elements have drawbacks that the voltage required for the elements to luminesce is as high as 100 V or higher, and that full-color displaying based on the three primary colors of red, green, and blue is difficult to attain because of difficulties in obtaining blue luminescence.
On the other hand, electroluminescent elements comprising organic materials have received attention from long ago. Although various investigations were made thereon, none of these was directed to full-scale practical use of organic electroluminescent elements because of the exceedingly low luminescent efficiencies thereof. However, in 1987, C. W. Tang et al. with Eastman Kodak Co. proposed an organic electroluminescent element having a laminated structure comprising two organic-material layers having functions respectively allotted thereto, i.e., a hole-transporting layer and a luminescent layer, and showed that this organic electroluminescent element attains a luminance as high as 1,000 cd/m.sup.2 or higher even at a voltage as low as 10 V or below [see, for example, U.S. Pat. No. 4,539,507 and C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 51, 913 (1987)]. Organic electroluminescent elements suddenly came to attract attention thereafter, and investigations are currently being made enthusiastically on organic electroluminescent elements having a similar laminated structure in which functions are allotted to the individual layers.
A conventional organic electroluminescent element is explained below with reference to FIG. 1, which is a view illustrating the constitution of the conventional organic electroluminescent element. In FIG. 1, numeral 1 denotes a substrate, 2 an anode, 3 a hole-transporting layer, 4 a luminescent layer, and 5 a cathode. As shown in FIG. 1, this conventional organic electroluminescent element comprises: a transparent or translucent substrate 1 made of, e.g., glass; an anode 2 which is a film of a transparent electroconductive material, e.g., indium tin oxide (hereinafter abbreviated as "ITO"), formed on the substrate 1 by sputtering, vapor deposition with resistance heating, etc.; a hole-transporting layer 3 made of, e.g., N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (hereinafter abbreviated as "TPD") formed on the anode 2 by vapor deposition with resistance heating, etc.; a luminescent layer 4 made of, e.g., 8-hydroxyquinoline aluminum (hereinafter abbreviated as "Alq") formed on the hole-transporting layer 3 by vapor deposition with resistance heating, etc.; and a cathode 5 which is a metal film having a thickness of from 100 to 300 nm formed on the luminescent layer 4 by vapor deposition with resistance heating, etc.
When a DC voltage or a direct current is applied to an organic electroluminescent element having the constitution described above, with the anode 2 and the cathode 5 as a positive electrode and a negative electrode, respectively, then holes are injected from the anode 2 through the hole-transporting layer 3 into the luminescent layer 4 and electrons are injected from the cathode 5 into the luminescent layer 4. In the luminescent layer 4, the holes recombine with the electrons and the resultant excitons shift from the excited state to the ground state, during which luminescence occurs. In the case where the luminescent layer in the above constitution contains Alq, green luminescence is obtained. It is theoretically possible to obtain luminescence of any desired color by employing an organic compound having a modified molecular structure. Consequently, organic electroluminescent elements can cope with full-color displaying and are promising future display elements having the advantage of low driving voltage. Although the above constitution has organic-compound layers of a laminated structure, i.e., a hole-transporting layer for transporting holes and a luminescent layer, other constitutions can be selected according to constituent materials. For example, use may be made of a constitution consisting of a luminescent layer alone, a three-layer structure composed of a hole-transporting layer, a luminescent layer, and an electron-transporting layer, or a constitution containing a layer which is a mixture of a luminescent layer with a hole-transporting layer or of a luminescent layer with an electron-transporting layer.
There also is a technique in which a luminescent layer partly doped with an organic compound dopant having a high fluorescent quantum yield is used and luminescence is taken out from the dopant (host-guest system). In this case, the material serving as the host is required to permit excitons resulting from its own luminescence to move smoothly in order to enable smooth exciton movement to the dopant. It is therefore necessary to select a host material satisfying requirements, for example, that the luminescent spectrum of the host material overlaps sufficiently with the excitation wavelengths of the dopant, and that the dopant is more susceptible to oxidation and reduction than the host material. There also is an element constitution in which energy barrier is taken in account in order to obtain luminescence not through exciton energy transfer from a guest. In this case, a host material which has the property of efficiently injecting or transporting holes or electrons to the dopant is selected. It is therefore possible to take out luminescence from a hole-transporting or electron-transporting layer having no luminescent region by incorporating a dopant thereinto. The dopant luminescence enables multiple colors ranging from blue to red, and a high-efficiency element can be provided by taking out the intense luminescence of dopants. In general, many materials for use as dopants have such a concentration quenching that these materials in a solid state show no clear fluorescence but in a dilute solution show intense luminescence. Such dopant materials are preferably used in a concentration not higher than several percents by mole. Consequently, materials used as thinner films have an advantage that there is a wider choice of materials because such materials are less required to have film-forming properties.
As described above, an organic electroluminescent element having a luminescent layer comprising an organic luminescent material can be made to have luminescence of any desired color by changing the molecular structure of the organic luminescent material. Furthermore, various high-efficiency luminescent elements based on the host-guest system have been proposed. However, such prior art elements are deficient in the satisfactory luminance properties and durability on a level suitable for practical use.